There is a rotary machine that rotates a turbine blade and an impeller fixed to a rotary shaft or compresses and scavenges a gas by the rotating turbine blade and impeller when the gas (air, flue gas) flows from a compressor, a supercharger, a steam turbine, a gas turbine, and the like. There may be a case where this kind of rotary machine makes some sounds when the gas flows. In this case, there may be a case in which a sound absorbing member is provided or a muffler to attenuate such sounds is installed in piping that makes sounds.
As such a muffler, there is a muffler including: a plate-shaped member formed with through-holes on a front surface; and a member that forms a space between the member and the plate-shaped member, and adapted to attenuate noise via the through-holes based on the principle of Helmhortz resonance (refer to Patent Literatures 1 and 2) Further, Patent Literature 3 discloses a structure in which sound waves transmitted inside a flow passage is reduced by forming recessed portions and bores inside the flow passage where a gas flows.